1. Field of the Invention
The present invention relates to a display control apparatus and method and, more particularly, to an apparatus and a method for controlling a display apparatus comprising display devices which contain display acting media, such as ferroelectric liquid crystals, for display refreshment so as to retain the refreshed state achieved by, for example, application of an electric field thereto.
2. Description of the Related Art
Cathode-ray tube displays (referred to as "CRTs" hereinafter) are widely employed in information processing systems in order to display information. However, because a CRT essentially requires a relatively large dimension (or thickness) substantially perpendicular to its display surface, further size-reduction of CRTs is not easy. Because CRTs have relatively large volumes, a CRT employed in an information processing system limits the freedom of use of the system, such as the locations for installation or portability of the system.
The freedom of use of information processing systems can be increased by employing liquid crystal displays (referred to as "LCDs" hereinafter) because an LCD allows reduction of the size (particularly, the thickness) of the entire display apparatus.
Some LCDs comprise liquid crystal cells containing ferroelectric liquid crystals (referred to as "FLCs" hereinafter), and thus are called FLC display (referred to as "FLCD" hereinafter). One of the characteristics of FLCDs is that the liquid crystal cells retain a display state for a substantially long time. Therefore, unlike the cycles of driving of CRT displays or liquid crystal displays of other types, the cycles of continuous refresh driving of an FLCD can be relatively long, and partial rewrite driving can be performed separately from the continuous refresh driving. Partial rewrite driving achieves a display change in a desired portion of a display frame. Therefore, the display panels of FLCDs can be made larger than the display panels of other types of liquid crystal displays.
FLCDs have liquid crystal cells which have substantially reduced thicknesses and contain long and narrow FLC molecules. The FLC molecules are bistable; more specifically, they are oriented in either the first stable state or the second stable state in accordance with the direction of an electric field applied thereto, and remain in the state after application of the electric field has been discontinued. Because of the bistability of FLC molecules, FLCDs have memory effect (or display-state retainability). FLCs and FLCDs are described in detail in, for example, Japanese Patent Application Laid-open No. 63-243919 and U.S. patent application Ser. No. 174,862, filed on Mar. 29, 1988.
However, the memory effect (or display-state retainability) of FLCDs causes problems. Because an FLCD latently retains an image which the FLCD displayed when it was last switched off, the FLCD displays the retained image, when powered on, for an instant before a starting image is displayed. Further, while being powered off, the image latently retained by an FLCD may be made visible by light incident thereon. The image thus latently retained by an FLCD, for example, an image carrying confidential information, could be viewed by someone who should not see such information.